ci: Set Go version consistently via .go-version
Ensure Go version is determined consistently for CI and Docker builds
rather than spread across several different files.
The intent is to eventually replace this with use of the `toolchain`
directive in Go 1.21.
We've noticed runners appearing to become resource-starved during heavy
CI traffic. While we should try to prevent this by limiting the
scanner's CPU consumption, increasing the runner size should help in the
interim.
* security: re-enable security scan release block
This was previously disabled due to an unresolved false-positive CVE.
Re-enabling both secrets and OSV + Go Modules scanning, which per our
current scan results should not be a blocker to future releases.
* security: run security scans on main and release branches
Michael Zalimeni